1. Field of the Invention
The present invention relates to an apparatus for detecting the presence or the absence of a recording medium in a printer.
2. Discussion of the Prior Art
There is known an apparatus for detecting the presence or the absence of a recording medium in a printer, which has a photo-electric sensor 4 disposed as shown in FIG. 2 such that the photo-electric sensor 4 faces a platen 2 which partially defines a feeding path for the recording medium in the form of a recording sheet 6. In this apparatus, the photo-electric sensor 4 generates a first output signal when the recording sheet 6 has not been fed to the platen 2, that is, when the recording sheet is absent at a printing position in the printer On the other hand, when the recording sheet 6 has been fed into position along the platen 2 by a known feeding system (not shown) such as a motor and a feed roller, that is, when the recording sheet 6 is present at the printing position, the photo-electric sensor 4 generates a second output signal.
Further, in the known apparatus for detecting the presence or absence of a recording medium, the photo-electric sensor 4 consists of a light emitting diode 4a and a photo-transistor 4b, as indicated in FIG. 3. The light emitting diode 4a is arranged so as to emit light toward the platen 2, and the photo-transistor 4b is positioned so as to receive the light reflected from the platen 2. The photo-transistor 4b supplies the first or second output signal to a comparator-detector circuit 8, which compares received output signal with a reference level, and detects the presence or the absence of the recording sheet 6 at the printing position, based on whether the level of the output signal is higher than the reference level.
In the known apparatus as described above, when the recording sheet 6 is absent at the printing position, the level of the first output signal of the photo-transistor 4b which corresponds to the absence of the recording sheet 6 is lower than the reference level with which the first output signal is compared by the comparator-detector circuit 8. Thus, the comparator-detector circuit 8 generates a signal indicative of the absence of the recording sheet 6 at the printing position. On the contrary, the level of the second output signal of the photo-transistor 4b corresponding to the presence of the recording sheet 6 is higher than the reference level The comparator-detector circuit 8 generates a signal indicative of the presence of the recording sheet 6 at the printing position.
The levels of the first and second output signals unavoidably vary or fluctuate due to a variation in the sensitivity of the photo-electric sensor 4, that is, chronological variations in the light intensity of the light emitting diode 4a and sensitivity of the photo-transistor 4b, and a time-wise or chronological variation in the reflectance of light of the surface of the platen 2. Further, the individual photo-electric sensors 4 for the individual printers have different sensitivities, due to the manufacturing errors from the nominal ratings. Also, there exists an inevitable error in the relative position of the photo-electric sensor 4 and the platen 2. Since these variations and errors cannot be ignored in assuring an accurate detection of the presence or absence of the recording medium, there has been proposed to limit the color of the recording sheet, or to provide the comparator-detector circuit 8 with a semi-fixed resistor 10, for adjusting the reference level, so as to minimize disadvantages caused by the above-indicated errors and variations, namely, to improve the detecting accuracy of the detecting apparatus.
In the case where the color of the recording sheet is limited as proposed above, however, the accurate detection is difficult when the recording sheet has images already printed thereon. In the case where the semi-fixed resistor 10 is used, the maximum amount of adjustment of electrical resistance of the resistor 10 cannot provide a sufficient amount of adjustment of the reference level, which covers the entire range of variations in the sensitivity of the photo-electric sensor 4 which may be three times as large as the adjustable range of the semi-fixed resistor 10.